Field of the Invention
This invention relates to molded thermosetting articles, and especially FRP (fiber-reinforced-plastic) articles, and more particularly, to coating compositions and methods for applying them to cover surface defects such as sink marks, porosity, microcracks and open (cracked) knit lines.
There has been increasing commercial interest in parts which are molded (generally compression molded) from resinous thermosetting materials. The materials most commonly used are resins such as polyester which are reinforced with fibers. The mixture of resin compound and reinforcing fibers, principally glass fibers, is generally formed into either a sheet molding compound (SMC) or a bulk molding compound (BMC). It is the SMC or BMC which is compression molded to form the desired part or substrate.
Among the commercially molded FRP parts, there are many which include ribs and bosses projecting downward from the underside of a relatively thin section on which it is desirable that the top surface be smooth and defect-free. A major disadvantage of molded parts having such a configuration is the appearance of surface depressions (commonly called "sink marks") on the top surface, opposite the ribs and/or bosses. It has been hypothesized that such sink marks are the result of uneven shrinkage of the resin material after curing of the part, even when the material is a "low shrink" or "low profile" polyester resin. The actual shrinkage occurs as the part cools, subsequent to the curing cycle, and manifests itself as a sink mark opposite a boss or rib, or any other thicker section, partly because it is at such thicker sections that a given shrinkage percent for a particular resin compound will result in a larger total amount of shrinkage.
In contrast to sink marks, which occur primarily over thickened sections, porosity may be found anywhere on the surface of the molded part, and is caused primarily by air entrapment and insufficient dispersion of certain substituents of the molding compound, such as the filler or reinforcing fibers. The major problems caused by porosity are associated with the painting of the part, and the durability of the paint. In particular, solvent from the paint can be trapped in the pores, ultimately causing a "paint pop."
A third type of defect (not necessarily a "surface defect") is microcracks which occur generally in areas of low glass content or high filler content and the result is a localized failure, caused ultimately by thermal or chemical shrinkage. These cracks which open to the surface of the part can cause paint problems similar to porosity.
Such surface defects are undesirable for two different but related reasons. First, many of the uses for molded FRP parts, especially automotive uses, require that the plastic part be capable of achieving the same smooth, glossy surface finish as painted sheet steel, in order for molded thermosets to be useable with, or compete with, i.e., replace, sheet steel. Secondly, when a molded FRP part has defects on a surface where a good finish is desired, it is necessary to repair the defects after the molding operation by filling them with a sealer and sanding the area after the sealer has hardened. In addition to these extra process steps which are required to repair defects, at least one reinspection is also generally required. If these extra manufacturing steps can be eliminated, not only would the cost of producing such a part be reduced, but also, increased competitiveness in price would open new markets to molded FRP parts.